The present invention relates to a method for anticorrosion phosphating and electro-dipcoating of components assembled from elements which are at least in part metallic, which consist in part of prephosphated zinc surfaces, in which the metallic elements of the component, which consist at least in part of prephosphated zinc surfaces, or the individual components have been subjected to heat treatment. This method overcomes the disadvantageous formation of irregularities during electro-dipcoating (“mapping”) of the phosphated component by means of an activating pretreatment with an acidic aqueous dispersion containing insoluble phosphates.
Various metallic materials and product forms are used in body construction in the automotive industry and in the manufacture of individual parts, for example doors. Some of these must be cut to size, shaped and then assembled to form the desired component. The materials and product forms used for this purpose are in the main “bright” metals, the surfaces of which do not comprise a coating which offers sufficient protection from corrosion or is suitable for the application of a coating system. Such “bright” surfaces also include, in addition to the actual metal surfaces, those which are provided with anticorrosion oils merely for transport or storage. Steel strip manufacturers, however, also supply prephosphated materials which can be immediately dipcoated on the particular OEM's premises before further coating layers are applied. The materials cut to size, shaped and assembled into the corresponding component for the production of an unfinished body accordingly not only comprise various metal surfaces but also have some surfaces which already comprise an initial passivation (phosphating) which provides protection from corrosion. One major material which is of significance in automotive production is here in particular prephosphated, galvanized steel strip, which is an essential component of the unfinished body and in component manufacture, for example of doors. When assembling the individual materials to form the finished component, the sheet metal parts are frequently joined not only by spot welding but also by continuous welding or clinching. Other methods such as riveting (e.g. for aluminum/steel joints) and adhesive bonding are increasingly frequently being used to assist welded joints, with adhesive bonding in particular becoming increasingly significant as the sole materially bonded joining method. Especially with adhesive joints, heat treatment of the elements preassembled to form the component is at present essential in order to ensure a durable adhesive bond between the individual elements (“pregelling”). During this “pregelling” of the applied adhesives, the prephosphated surfaces of the component are exposed to elevated temperatures which permanently modify the chemical and physical characteristics of the phosphate layer, in particular of a zinc phosphate layer. The modified properties of the heat-treated prephosphated components then have an impact on the subsequent anticorrosion treatment and specifically on electro-dipcoating. It is, for example, observed that electro-dipcoating on the prephosphated surfaces results in different coating layer weights than on those surfaces which were not subjected to phosphating until they reached the OEM's premises. These differences in the quality of dipcoating, which are visible on the assembled and heat-treated component, are coating defects and are described as “edge mapping”. This “mapping” also has an effect on further top coating layers which are applied, meaning that the non-uniform appearance still remains visible once the complete coating structure has been applied. If the assembled and heat-treated component also comprising prephosphated surfaces is phosphated by spraying, the “mapping” after electro-dipcoating is also particularly apparent in the form of “curtaining”, which reproduces the initial wetting pattern, typical of spraying, of the prephosphated areas with the phosphating solution.
The problem of “mapping” is thus a current problem in particular in automotive manufacture and has not previously been adequately addressed and handled in the prior art.
EP 0977908 accordingly discloses an activating pretreatment solution for metallic surfaces prior to zinc phosphating which contains
    (a) ammonium and/or alkali metal salts, which preferably constitute phosphates,    (b) dispersed phosphates of di- and/or trivalent metal cations and    (c) microparticulate oxides for stabilizing the dispersion.According to the teaching of EP 0977908, the dispersion can be used over a wide pH range from 4 to 13, the particularly preferred pH range for maximally effective activation of the metal surfaces being between 7.5 and 8.5. A person skilled in the art cannot, however, infer from this disclosure any teaching as to avoiding “mapping” on heat-treated prephosphated areas of metallic components in subsequent electro-dipcoating.